I agree with all of the others,,,take it to the shop! I also work at a mtn bike park, and our xs bikes do not have that problem. I really think it's simply not set up quite right. Regardless of what size it is, rubbing on the BOTTOM ( not the side plate) of the derailleur is a height/compression issue and is something that is quickly fixed. Whether it's Shimano or Sram is also not the issue.

It sounds like you have a great bike, and although there is an adjustment phase in getting used to the size... in the long run, it truly sounds like it's a better size for you. Mountain bikes do not have as many options for sizing as road bikes, but hopefully one day they will... it's certainly getting better all the time.

Have fun!